Congratulate your opponent
Sometimes you have to help your partner a bit to feel that he won the negotiations. So, after the talks you should congratulate him the successful transaction. Praise him for his perfect negotiation skills and a way in which he deals with you. You can say, for example something like: "I am truły impressed you madę me sell this product below the recommended price.”
Do not grudge your partner compliments and approval, which do not cost a thing and, in the long haul, can result in morę benefits than you think.